1. Field of the Invention
The present invention refers to a coffee brewing machine, particularly an espresso coffee brewing machine, comprising a container for fresh cold water, a boiler for heating the fresh cold water, a brewing chamber adapted to receive coffee powder, a pump for feeding the fresh cold water from the container to the boiler and therefrom into the brewing chamber, a piston fitting into the brewing chamber and being movably mounted such that it can be moved into the brewing chamber to compress the coffee powder contained in the brewing chamber and removed from the brewing chamber, and drive means for driving the piston into the brewing chamber and for retracting it out of the brewing chamber.
2. Prior Art
Such coffee brewing machines are well known in the art for a long time and are used, even if they are called "espresso machine", both for brewing espresso coffee as well as normal coffee. Automatically operating coffee machines of this kind usually comprise an electric motor for driving a piston into a brewing chamber containing coffee powder and for retracting the piston out of the brewing chamber when the coffee has been brewed. Once the coffee powder contained in the brewing chamber is compressed, pressurized hot water is fed into the brewing chamber; the hot water penetrates the compressed coffee powder and escapes from the brewing chamber as coffee.
As the piston has to be displaced in two opposite directions, on the one hand toward the brewing chamber and into its interior for compressing the coffee powder, and on the other hand in the opposite direction, out of the interior of the brewing chamber to enable the used coffee powder to be removed from the brewing and to enable fresh coffee powder to be filled into the brewing chamber, a complicated and expensive linkage construction between electric motor and piston had to be used. Such linkage construction is required to transform the rotational motion of the electric motor into a linear displacement of the piston in two opposite directions, since the electric motors used up to now are electric AC motors.
The aforementioned linkage construction for transforming the rotational motion in a linear motion has three basic disadvantages:
1. It is expensive and increases the price of the coffee machine.
2. It is liable to give trouble and it is subjected to wear, with the result that the expenditure of servicing is quite high.
3. The weight of the coffee machine is increased.
For these reasons, at first sight, it could have been obvious to use an electric DC motor for the displacement of the piston because a DC motor can be operated in two opposite senses of rotation without problems However, the provision of an electric DC motor involves other disadvantages:
1. In order to operate an electric DC motor, a DC power supply must be provided
2. Electric DC motors which come into consideration as far as the price is concerned usually have an operating voltage of less than 80 volts, often 48 volts. Thus, the mains voltage of 220 or 110 volts must be transformed down to the operating voltage of the DC motor and, further, must be rectified. The provision of a suitable transformer, however, would ruin the advantages in price and weight gained by the use of an electric DC motor.
A further disadvantage with known espresso coffee machines is that the coffee often is quite cold when it has been prepared, particularly if an espresso coffee is prepared (small amount of hot water) and if the coffee cups are not preheated.